1. Field of Invention
This invention relates to a luminescent material and element and, more particular, to an organic electroluminescent (EL) material and an organic EL element.
2. Related Art
Following the advances in electrical technology, light and high efficiency displays, such as liquid crystal displays (LCD), are well developed. However, the LCD has several drawbacks: the narrow viewing angle, the response time which is not fast enough to display high-speed animation, and the increased power requirement for driving the panel. Moreover, a large panel cannot be easily manufactured in LCD structures.
Compared to the LCD, organic light-emitting diodes are self-emissive, have a full viewing angle, are high power efficient, are easily manufactured, are low cost, have a fast response time, and are full color. Therefore, organic light-emitting diodes could be the major flat display and light source, including use as special light sources and for normal illumination, in the future.
Referring to FIG. 1, an organic light-emitting diode 1 includes a substrate 11, a first electrode 12, an organic EL layer 13, and a second electrode 14. When applying a direct current to the diode 1, holes are injected from the first electrode 12 into the organic EL layer 13 while electrons are injected from the second electrode 14. Based on the applied voltage, the holes and electrons are moved in the organic EL layer 13, and are combined to generate excitons. The excitons can excite organic EL materials of the organic EL layer 13, so that the excited organic EL materials emit light to release energy.
Those skilled in the art should know that organic light-emitting diodes utilize the self-emissive of organic functional materials to achieve the objective of displaying.
The organic compound of the organic EL layer has been long studied. For example, W. Helfrish, Dresmer, Williams, et al. succeeded in emission of blue light using anthracene crystals (J. Chem. Phys., 44, 2902 (1966)). Vincett, Barlow, et al. produced a light emitting device by a vapor deposition method, using a condensed polycyclic aromatic compound (Thin Solid Films, 94, 171 (1982)). However, only a light emitting device low in luminance and luminous efficiency has been obtained.
In 1987, C. W. Tang and S. A. Van Slyke disclosed an organic EL layer structure having an organic thin film and a transporting thin film. The transporting thin film is a hole transporting layer or an electron transporting layer. It is reported that the maximum luminance provided is more than 1,000 cd/m2 and an efficiency of 1 lm/W (Appl. Phys. Lett., Vol. 51, 913 (1987)).
After that, scientists developed another organic EL layer structure having three layers to decrease driving voltage of the diode and to increase the maximum luminescence. In this case, the organic EL layer structure having a luminescent layer, a hole transporting layer, and an electron transporting layer.
It is also reported that a distyrylbenzene compound well known as laser dye exhibits high fluorescent properties in the region of blue to blue green, and a light emitting material using the distyrylbenzene compound in a single layer form emits EL light of about 80 cd/m2 (European Patent 0319881). In the recent 10 years, Idemitsu Kosan Co. disclosed derivatives of distyrylbenzene compounds and has many granted patents such as U.S. Pat. Nos. 5,121,029, 5,126,214, 5,130,603, 5,516,577, 5,536,949, 6,093,864, WO 02/20459, and et al. In addition, the styrylbenzene compound and its derivatives are reported in Synthetic Metal 121 (2001) 1661, Synthetic Metal 121 (2001) 1665, Appl. Phys. Lett. 67 (26) 1995, Materials Science, and Engineering B85 (2001) 126.
Although derivatives of distyrylbenzene compounds are well studied utilizing in organic EL material and EL element, there are still several drawbacks such as low luminance and emitting efficiency, high driving voltage, color impurity, and et al. For example, as disclosed in U.S. Pat. No. 5,130,603, N,N′-diphenyl-N,N′-bis-(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine (TPD) is used in a hole transporting layer, and 2,5-bis(2,2-di-p-tolyvinyl)xylene (DTVX) is used in a luminescent layer. When applying 5 volts, the luminance of the EL element having TPD and DTVX is 300 cd/m2, and the luminescent wavelength of the element is 486 nm. When applying 7 volts, the maximum luminance of the EL device is 1,000 cd/m2. In addition, as disclosed in U.S. Pat. No. 5,536,949, TPD is used in the hole transporting layer, 4,4′-Bis(2,2-diphenylvinyl)biphenyl (DPVBi) is used in the luminescent layer which doped with 4,4′-Bis[2-{4-(N,N-diphenylamino)pheny}vinyl]biphenyl (DPAVBi), and 8-hydroxyquinoline is used in the electron transporting layer. In this case, when applying 8 volts, the luminance of the EL element is 400 cd/m2, and the luminescent wavelength of the element is 494 nm. In U.S. Pat. No. 6,093,864, the EL element has similar properties as mentioned above. In this case, derivatives of distyrylbenzene compounds are formed in the organic EL element by the evaporation method. However, these molecules are thermally unstable, so that they could be thermally degraded during the testing of the manufacturing processes.
Alternatively, some scientists have disclosed that tris-styrylbenzene compounds (Synthetic Metal 121 (2001) 1661) or tetrakis-styrylbenzene compounds (Synthetic Metal 121 (2001) 1665) can be used in the luminescent layer. In practice, however, the luminous efficiency of the organic EL element having those compounds is unsatisfactory, and it is difficult and complex to manufacture an organic EL element having those compounds.
Therefore, it is an important objective of the invention to provide an organic EL material and EL element that can improve luminance, emitting efficiency, driving voltage, and color impurity. Furthermore, the organic EL material and EL element of this invention can also improve thermal stability to prevent thermal degradation.